This invention relates to a multi-part rubber/elastomer seal system for a proton exchange membrane (PEM) fuel cell reactant gas manifold.
A basic fuel cell comprises an anode electrode spaced apart from a cathode electrode with an electrolyte disposed therebetween in a compartment formed between the two electrodes; each electrode also includes a catalyst layer on the electrolyte side thereof. On the non-electrolyte side of the anode electrode is a reactant gas chamber for carrying a fuel, and on the nonelectrolyte side of the cathode electrode is a reactant gas chamber for carrying an oxidant. The electrodes are constructed so that the gas diffuses therethrough and comes into contact with the electrolyte in the catalyst layer thereby causing a well-known electrochemical reaction whereby hydrogen ions and electrons produced at the anode travel from the anode electrode through, respectively, the electrolyte and the external circuit to the cathode electrode where they react with oxygen to produce heat and water. This flow of electrons is the electric current produced by the cell.
In a fuel cell power plant a number of fuel cells are connected electrically in series through plates separating adjacent cells, thereby forming a cell stack assembly (CSA). These plates in combination with the electrodes adjacent thereto, generally define the reactant gas passages or chambers. The CSA is provided with external fuel and oxidant manifolds for simultaneous supply to and exhaust of gases from the individual cells. Accordingly, it is necessary to provide manifold-to-CSA seals to prevent leakage of the gases involved in the operation of the fuel cell stack assembly. Also, during operation of the CSA, the CSA and the manifold-to-CSA seals undergo compressive creep. Thus, since the CSA is made up of a series of cells, the edges of which are not aligned, and the surface of the stack is therefore rough, compressive creep of the stack and the manifold-to-CSA seals results in increased reactant leakage.
Seals for phosphoric acid fuel cells as shown in commonly owned U.S. Pat. No. 4,774,154, have successfully employed a composition which includes a high fluorine content fluorinated hydrocarbon elastomer, a carbon black filler, azodicarbonamide blowing agent and blowing agent promoter, and an epoxy or inorganic oxide acid acceptor. However, this composition cures in the range between 176xc2x0 C. and 204xc2x0 C. (350xc2x0 F. and 400xc2x0 F.). Since the proton exchange membrane is generally damaged above about 150xc2x0 C. (about 300xc2x0 F.), curing the prior seal compound would destroy a PEM fuel cell.
PEM fuel cells known to the prior art have utilized closed cell, neoprene rubber foam to accommodate the uneven surface of the side of the fuel cell stack. However, this material exhibits poor springback characteristics, resulting in increased leakage as a function of time and a requirement to continuously tighten the manifolds.
Objects of the invention include a manifold seal system for a PEM fuel cell stack assembly which can be effected at stack temperatures not exceeding 150xc2x0 C. (300xc2x0 F.), which is stable in the presence of hydrogen, oxygen and water, which has low creep and which is very compressible.
According to the present invention, a manifold seal system for a PEM fuel cell comprises at least two parts, including one or two layers of silicone rubber applied to provide smooth bridges between one stack end plate (sometimes referred to as xe2x80x9cpressure platexe2x80x9d), and the opposite end plate, there typically being one such bridge along each edge of the stack, another part comprising an elastomer adjacent to the reactant manifold, said elastomer having a low compression set (low creep), that is, below 25%, and a low compression modulus (being very springy), that is, capable of compressing about 25% under loads of less than 50 psi.
In one embodiment employing low temperature curing silicone rubber, an additional part comprises a rubber strip in contact with the end plates and in contact with the silicone rubber bridges; a bead of silicone rubber as used on the bridges may also be added under the rubber strip along the end plates, if required for leveling purposes. In accordance with one embodiment of the present invention, the cell bridges may comprise a two-part silicone rubber capable of curing at room temperature or a one-part silicone room temperature vulcanizing silicone rubber adhesive. In one embodiment, the bridges comprise two different room temperature curing silicone rubbers: first, a controlled quantity of a compatible liquid silicone rubber, having low shrinkage characteristics, a low viscosity and self-leveling capability is applied to the stack surface; this is followed by a higher viscosity, liquid rubber to fill the larger voids and crevices of the stack surfaces and to act as a bonding agent for the strip of pre-cured solid rubber.
In another embodiment, a bead of heat-cured silicone rubber sealant is applied between and on the end plates and cured with a heated platen, to a thickness determined either by shims or by volume/pressure control.
The elastomer may comprise closed cell silicone rubber foam of low to medium density, or closed cell silicone rubber sponge, but the preferred embodiment is a molded silicone rubber gasket. As used herein, xe2x80x9clow density foamxe2x80x9d means a foam that exerts a pressure of about 2-5 psi at a deflection of 25% while xe2x80x9cmedium density foamxe2x80x9d exerts a pressure of about 10-20 psi at a deflection of 25%. A low density foam also has a bulk density of about 0.15-0.25 grams per cubic centimeter and a medium density foam has a bulk density of 0.25-0.35 grams per cubic centimeter. The compression set of the seal material should be less than 35% after 72 hours at 190xc2x0 F.; this is defined as xe2x80x9clow compression setxe2x80x9d herein. The preferred range of thickness is between {fraction (1/16)} of an inch and {fraction (3/16)} of an inch.
The invention provides an effective seal to the fuel cell for all pressure differentials experienced by the seal during fuel cell operation, without leaking, and does not require periodic tightening of a manifold to maintain the seal over time. The sealing system of the invention allows easy removal of the manifold, should it be required.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.